In a conventional refinery flow scheme, crude oils are subjected to an atmospheric distillation step designed to remove light materials such as gas oils, kerosenes, gasolines, straight run naphtha, etc. The residue from the atmospheric distillation step is then typically subjected to a distillation step at a pressure below atmospheric. The vacuum distillation step produces a vacuum gas oil distillate and vacuum reduced residual oil which often contains asphalts, resins, and heavy hydrocarbonaceous oil components. The additional steps involved in upgrading vacuum resid are important in that these oils are often used as the feedstock for a lubricating oil manufacture.
There are a number of processes used for recovering the useful portions of various asphaltic petroleum residual oils. Many processes involve the extraction of the oil with a deasphalting solvent such as propane, and thereafter separating and recovering the oil components from the solvent. In U.S. Pat. No. 2,950,244, a process for the extraction of petroleum residue containing asphalt is disclosed. The solvent utilized is a liquefied normally gaseous solvent, such as propane, which is maintained at a temperature between 100.degree. and 200.degree. F. and a pressure sufficient to maintain the solvent in a liquid phase.
Variations of the deasphalting process using propane or similar short chain aliphatics as solvents are shown in U.S. Pat. No. 2,669,538 to Yurasko et al; U.S. Pat. No. 3,516,928 to King et al, issued June 23, 1970; U.S. Pat. No. 4,017,383 to Beavon, issued Apr. 12, 1977; and U.S. Pat. No. 4,201,660 to Szosel, issued May 6, 1980. King et al additionally suggests that carbon dioxide and ammonia are, under certain circumstances, equivalent to the disclosed lower alkanes, alkenes and their halogenated derivatives.
Although propane is often used in deasphalting operations, other solvents have been suggested. In U.S. Pat. No. 4,054,512, an asphalt-containing mineral oil is deasphalted by contacting the oil with liquid hydrogen sulfide. The use of liquid neopentane, at a temperature between 0.degree. and 250.degree. F., as the deasphalting solvent is shown in U.S. Pat. No. 3,334,043. In U.S. Pat. No. 2,337,448, heavy residual oil is deasphalted by using a solvent selected from the group ethane, ethylene, propane, propylene, butane, butylene, isobutane, and mixtures thereof.
Multi-stage solvent extraction techniques involving the use of one or more solvents are also known. In U.S. Pat. No. 3,658,665 a heavy oil is subjected to a two-stage extraction process. In the first stage, the heavy oil is contacted with a solvent and the mixture is thereupon subjected to additional solvent in a second zone. The second zone is maintained at a higher temperature than is the first solvent stage. In U.S. Pat. No. 4,017,383, a multi-stage deasphalting process is shown in which the recovery of the solvent from the extracted hydrocarbon is effected in a series of two or more pressure stages. The solvents are liquefied low molecular weight hydrocarbons, such as propane or isobutane.
Additionally there are a number of processes which use multiple or mixed solvents to deasphalt various oils. For instance, in U.S. Pat. No. 2,188,051 to Lantz, issued Jan. 23, 1940, the oil is contacted with solvent preferably containing at least 75% of paraffinic hydrocarbons having less than 7 carbon atoms. Ethane, propane, butane, isobutane, pentanes, and hexanes constitute the preferred solvents. The hydrocarbon solvent is mixed with a carbon dioxide for later addition to the oils to be extracted.
In U.S. Pat. No. 2,631,966 to Francis, issued Mar. 17, 1953, a process using liquid carbon dioxide and a variety of other solvents are used to separate various portions of the hydrocarbon feed. The solvents are divided into two classes. The first class is one whose members are completely miscible with liquid carbon dioxide but incompletely miscible with the oil to be extracted. The second class involves solvents which are incompletely miscible with carbon dioxide and also incompletely miscible with the oil to be extracted. Both sets of solvents are further defined to be those which do not form a solid salt with carbon dioxide at temperatures of 20.degree. C. and higher. Included in the first class of solvents are such compounds as dichlorodiethyl ether, isopropanol, betaethoxy ethanol, diethylene glycol monoethyl ether and the like. Members of the second class of solvents include aniline, o-chloroaniline, m-chloroaniline, cresols and the like. The process generally includes the steps of adding the mixture of carbon dioxide and solvent to the oil and removing the carbon dioxide at various stages to effect separation of various types of hydrocarbon oils. U.S. Pat. No. 2,646,387 also to Francis, issued July 21, 1953, suggests an improvement to the process discussed above. The improvement identifies a method of recovering the solvent from the hydrocarbon oil by addition of liquid carbon dioxide to the solvent-oil mixture so as to form a solvent-carbon dioxide phase.
U.S. Pat. No. 4,179,362 to Irani et al, issued Dec. 18, 1979, suggests the separation of petroleum fractions into aromatic-rich and paraffinic-rich hydrocarbon streams by the use of methanol/water mixtures. The paraffin-rich stream is recovered as raffinate and aromatic-rich stream as the extract. After the extraction step, additional water is added to the extract and raffinate streams where the water acts as an antisolvent to effect separation of the hydrocarbon from the solvent. The water and methanol are then separated either by flash distillation or by using supercritical carbon dioxide as an extraction solvent. In U.S. Pat. No. 4,191,639 to Audeh et al, issued Mar. 4, 1980, hydrocarbon oils such as residual petroleum oils are deasphalted and demetallized by contact with a liquid mixture of at least two of the components selected from hydrogen sulfide, carbon dioxide, and propane.
None of the above references are believed to suggest a process in which a heavy oil is admixed with a solvent which is completely miscible with the oil at low treat ratio followed by treatment of the resulting single phase with carbon dioxide to form separate phases.